Modelling magnetic anisotropy of single-chain magnets in |<i>d</i>/<i>J</i>| ≥ 1 regime
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Single-molecule magnets (SMMs) with single-ion anisotropies comparable to exchange interactions <i>J</i> between spins have recently been synthesised. Here, we provide theoretical insights into the magnetism of such systems. We study spin chains with site-spins, <i>s</i> = 1, 3/2 and 2 and strength of on-site anisotropy comparable to the exchange constants between the spins. We find that large on-site anisotropies lead to crossing of the states with different <i>M</i><sub><i>S</i></sub> values in the same spin manifold to which they belong in the absence of anisotropy. When on-site anisotropy is increased further, we also find that the <i>M</i><sub><i>S</i></sub> states of the higher energy spin states descend below the <i>M</i><sub><i>S</i></sub> states of the ground spin manifold. Giant spin in this limit is no longer conserved and describing the axial and rhombic anisotropies of the molecule, <i>D</i><sub><i>M</i></sub> and <i>E</i><sub><i>M</i></sub>, respectively, is not possible. However, the giant spin of the low-lying large <i>M</i><sub><i>S</i></sub> states is very nearly an integer and, using this spin value, it is possible to construct an effective spin-Hamiltonian and compute the molecular magnetic anisotropy constants <i>D</i><sub><i>M</i></sub> and <i>E</i><sub><i>M</i></sub>. We report effect of finite sizes, rotations of site anisotropies and chain dimerisation on the effective anisotropy of the spin chains.
